1. Field of the Invention
The present invention relates to a full color surface discharge type plasma display panel (i.e., a flat display panel having a memory function, such as an AC-type PCP) and to a method and apparatus for driving the same to allow gradation, i.e., a gray scale, of its visual brightness for each cell. More specifically, the present invention relates to a full color ac plasma display device which is high in resolution and brightness of display, such that it is adaptable for use as a high quality display, such as a high definition TV, and can be used in daylight.
2. Description of the Related Art
A plasma display panel (PDP) has been considered the most suitable flat display device for a large size, exceeding over 20 inches, because a high speed display is possible and a large size panel can easily be made. It is also considered to be adaptable to a high definition TV. Accordingly, an improvement in full color display capability in plasma display panels is desired.
In the past, two electrode type dc and ac plasma display panels have been proposed and developed. Also, a surface discharge type ac plasma display panel, among other plasma display panels, has been known to be suitable for a full color display.
For example, a surface discharge type ac plasma display panel having a three electrode structure comprises a plurality of parallel display electrode pairs formed on a substrate and a plurality of address electrodes perpendicular to the display electrode pairs for selectively illuminating unit luminescent areas. Phosphors are arranged, in order to avoid damage by ion bombardment, on the other substrate facing the display electrode pairs with a discharge space between the phosphor and the display electrode pairs and are excited by ultra-violet rays generated from a surface discharge between the display electrodes, thereby causing luminescence. See for example, U.S. Pat. No. 4,638,218 issued on Jan. 20, 1987 and No. 4,737,687 issued on Apr. 12, 1988.
The full color display is obtained using an adequate combination of three different colors, such as red (R), green (G) and blue (B), and an image element is defined by at least three luminescent areas corresponding to the above three colors.
Conventionally, an image element is composed of four subpixels arranged in two rows and two columns, including a first color luminescent area, for example, R, a second color luminescent area, for example, G, a third color luminescent area, for example, G, and a fourth color luminescent area, for example, B. Namely, this image element comprises four luminescent areas of a combination of three primary colors for additive mixture of colors and an additional green having a high relative luminous factor. By controlling the additional green area independent from the other three luminescent areas, an apparent image element number can be increased and thus an apparent higher resolution or finer image can be obtained.
In this arrangement of four subpixels, two pairs of display electrodes cross an image element, i.e., each pair of display electrodes crosses each row or column of subpixels, which is apparently disadvantageous in making image elements finer.
If the image elements are to be finer, formation of finer display electrodes becomes difficult and the drive voltage margin for avoiding interference of discharge between different electrode lines becomes narrow. Moreover, the display electrodes become narrower, which may cause damage to the electrodes. Further, a display of one image element requires time for scanning two lines, which may make a high speed display operation difficult because of the frequency limitation of a drive circuit.
The present invention is directed to solve the above problem and provide a flat panel full color surface discharge type plasma display device having fine image elements.
JP-A-01-304638, published on Dec. 8, 1989, discloses a plasma display panel in which a plurality of parallel barriers are arranged on a substrate and luminescent areas, in the form of strips defined by the parallel barriers, are formed. This disclosure is, however, directed only to two electrode type plasma display panels, not to a three electrode type plasma display panel in which parallel display electrode pairs and address electrodes intersecting the display electrode pairs are arranged and three luminescent areas are arranged in the direction of the extending lines of the display electrode pairs as in the present invention.
The present invention is also directed to a plasma display panel exhibiting a high image brightness at a wide view angle range. In this connection, U.S. Pat. No. 5,086,297 issued on Feb. 4, 1992, corresponding to JP-A-01-313837 published on Dec. 19, 1989, discloses a plasma display panel in which phosphors are coated on side walls of barriers. Nevertheless, in this plasma display panel, the phosphors are coated selectively on the side walls of barriers and do not cover the flat surface of the substrate on which electrodes are disposed.
The present invention accordingly is directed to improving upon the structure of a full color, surface discharge type plasma display device so as to provide finer image elements and thus improved resolution and also increased brightness of the display while being compatible with and not introducing excessive speed (frequency) requirements on a driving circuit but providing high speed display operations. An improved display gradation, as high as 256 grades, or levels, is also required in combination with the improved structure so as to improve the picture quality of the display and thereby to obtain a picture quality at the level of high-definition television, as an example.
There have been proposed heretofore various methods for increasing the selective gradation of the display brightness, such as disclosed in Japanese Patent Publication 51-32051 or Hei 2-291597, wherein a single frame period of a picture to be displayed is divided, with respect to time, into plural subframes (SF1, SF2, SF3, etc.), and wherein each subframe has a specific time length for lighting a corresponding cell, so that the visual brightness of the cell is weighted. A typical prior art method to provide such gradation of visual brightness of a display is schematically illustrated in FIG. 1, wherein, after cells on a single horizontal line (simply referred to hereinafter as a line) Y.sub.1 are selectively written, i.e., addressed, cells on the next line Y.sub.2 are then written. A structure of each subframe SFn on each scanned line, as employed in an opposed-discharge type PDP panel, is shown in FIG. 2, where there are drawn voltage waveforms, as are applied across the cells on horizontal lines Y.sub.j, Y.sub.j+1, Y.sub.j+2, . . . , respectively. Each subframe is provided with a write period CYw (or address period) during which a write pulse Pw, an erase pulse Pf and sustain pulses Ps are sequentially applied to the cells on each Y-electrode, and a sustain period CYm during which only sustain pulses are applied.
The write pulse generates a wall charge in the cells on each line and the erase pulse Pf erases the wall charge. However, for a cell to be lit, a cancel pulse Pc is selectively applied to the cell's X-electrode X.sub.f concurrently with application of the erase pulse, so as to cancel the erase pulse Pf. Accordingly, the wall charge (see FIG. 10) remains only in the cell applied with the cancel pulse Pc, that is, where the cell is written. Sustain pulses Ps are concurrently applied to all the cells; however, only the cells having the wall charge are lit.
Gradation of visual brightness, i.e., a gray scale, is proportional to the number of sustain pulses that light the cells during a frame. Therefore, different time lengths of sustain periods CYm are allocated to the subframes in a single frame, so that the gradation is determined by an accumulation of sustain pulses in the selectively operated subframes, the subframes having respective, different numbers of sustain pulses.
A problem in such prior art drive methods exists, in that a second (i.e., a subsequent) subframe must wait for the completion of a first (i.e., a preceding) subframe for all the lines, creating an idle period on each line. Therefore, if the number of the lines m=400 per frame and for 60 frames per second to achieve a brightness gradation of 16 grades (n=4), the time length T.sub.SF allowed for a single subframe period becomes as short as about 10 .mu.g as an average, because T.sub.SF .times.60.times.400.times.4=1 sec. For executing the write period and the sustain period in such a short period, the driving pulses must be of a very high frequency. For example, in the case where the numbers of sustain pulses are 1, 2, 4 and 8 pairs in the respective subframes to achieve 16 grades, a frequency of the driving pulses must be as high as 360 kHz, as derived from: EQU freq.=(1+2+4+8).times.60.times.400=360.times.10.sup.3 Hz.
Such a high frequency drive circuit consumes a high level of power and allows less margin in its operational voltage due to the corresponding minimal storage time for the wall charge, particularly in an AC type PDP. Moreover, the high frequency operation, such as 360 kHz, may cause a durability problem of the cell. Therefore, the operation frequency cannot be easily increased, resulting in a difficulty in achieving the desired gradation.
Furthermore, in the above prior art method, a write period CYw of a line must be executed concurrently with a sustain period CYm of another line. This fact causes another problem in that the brightness control, for example, the gradation control to meet gamma characteristics of a human eye, cannot be desirably achieved.